A shock absorber which changes a damping force by causing a magnetic field to act on a flow passage in which the magnetorheological fluid passes and changing an apparent viscosity of magnetorheological fluid is known as a shock absorber installed in a vehicle such as an automotive vehicle. JP2008-175364A discloses a magnetorheological fluid shock absorber in which magnetorheological fluid passes in a flow passage formed between a piston core having a coil wound on the outer periphery thereof and a piston ring arranged on the outer periphery of the piston core when a piston assy including the piston core and the piston ring slides in a cylinder.